A heat-treated billet of U.sub.3 Si has a hardness of about 200 VPN, a ductility of 1%, a compressibility of about 20% at room temperature, and is too tough to be crushed or comminuted easily.
Until now, a milling machine or a lathe was used for coarse comminution to chips, or a large press (over 200 tons) was used for crushing. Chips of U.sub.3 Si are highly combustible due to its high oxidation receptive characteristics. Thus, it is necessary to machine comminute under a sufficient amount of cutting fluid to substantially prevent oxidation. Impurities gather in chippings from the cutting fluid, which necessitates rinsing and drying as extra processes. In addition, crushing by means of a large press carries with it the danger of fire, and a safety facility is needed to protect the environment from fumes and/or fine air-borne particles. Coarse particles crushed in presses also include impurities introduced from the press die, which requires further rinsing and drying.
Pulverizing the chips into fine particles is usually done in a hammer mill, impact mill, shatter box, or vibration mill. In any case, impurities are gathered from the part of each machine which is worn by the collision of the high speed comminuted particles. This equipment should be surrounded by argon or nitrogen since the fine particles of uranium alloys may explosively oxidize in open air.
During comminution, heavy, complicated equipment is needed, along with a series of tedious and laborious processes. Ferrous impurities, mainly due to the wear of machine parts, are suitably removed by magnetic separation. Very often, the uranium fuel components are removed at the same time since the particles containing ferrous components often carry some uranium with them. This introduces inefficiencies in fabrication and necessitates a waste treatment process for the magnetically separated particles. It is concluded that if possible, comminution by these techniques should be avoided.
The chemical composition of a cast ingot differs as a function of position in the ingot. U.sub.3 Si alloys are melted and cast in high frequency induction furnaces or in arc furnaces. The microstructure of the cast ingot shows typical dendrites when the cooling rate is fast. The matrix formed is uranium containing silicon up to its solubility level with U.sub.3 Si.sub.2 dendrites as the primary phase. As the cooling rate becomes slower, the primary U.sub.3 Si.sub.2 phase transforms to a particle shape, showing faceting planes. Such ingots show a great negative compositional segregation of about .+-.0.1% silicon such that the silicon is higher in concentration in the upper portion of the ingot than it is in the lower portion. Furthermore, the silicon content varies microscopically due to dendritic growth. These microscopic and macroscopic segregations affect the following peritectoid heat treatment and the homogeneity of fuel components produced therefrom.
The cast ingot contains microstructures of U.sub.3 Si.sub.2 and U.sub.ss (uranium matrix with solution of silicon up to solubility) as mentioned above. U.sub.ss is a harmful material in the reactor, due to hot spot effects and dimensional instability. U.sub.3 Si.sub.2 is stable but has a lower loading density than U.sub.3 Si. The ingot structure of U.sub.3 Si.sub.2 and U.sub.ss is therefore changed into U.sub.3 Si by a heat-treatment as shown below. EQU U.sub.3 Si.sub.2 +U.sub.ss --2 U.sub.3 Si (peritectoid reaction)
In conventional processes, this takes 72 hours at 800.degree. C. This can be shortened if powders are heat-treated because of the larger surface area of powders. It was verified by the inventor that the degree of the peritectoid reaction in a powder heat-treatment depends on the compositional homogeneity of each powder. This compositional homogeneity is increased up to its maximum by this invention.